The continuous coring method is the utilisation of modified small mining (or other) rigs capable of continuous coring using wireline retrievable core barrels, coupled with on-site evaluation of the cores (which is now available commercially).

Potential benefits (specific to continuous coring)

·The quantity and quality of information that could be obtained through coring in a series of cheap, "throwaway" wells, could lead to exploration strategies involving the close integration of drilling into the information and evaluation stages.

·Continuous coring can accelerate the building of a geological model which may assist with quantification of the prospect and in identifying the potential scope.

Potential disadvantages

·Some of the drilling contractors who operate mining rigs have little experience of drilling for hydrocarbons in sedimentary formations.

·The surface and subsurface drilling equipment and techniques have been optimised for mineral exploration drilling in hard rock. This can lead to problems in, well control, shutting in on false kicks, drilling in active shales, slow penetration rates, stuckpipe, drillstring vibration failure, certification and explosion proofing of electrical equipment, etc.

·Coring still has to be terminated when high overpressures are encountered due to the limitations of the mud systems being used. Continuous coring has a maximum proven depth of about 3000 metres.

·Coring is slower than destructive drilling because the core needs to be recovered by wireline at intervals.

·Continuous coring has been used on land and from swamp barges but has not yet been used for hydrocarbon exploration from floating rigs with significant heave.

Costs

Total well cost savings of 50%, relative to coring with a conventional rig, have been reported and accumulate mostly in the mobilisation of the rig and consumables, and in the preparation of smaller locations. In an established area with existing rigs, continuous coring is usually considerably more expensive than conventional drilling due to the slower progress and costs of mobilising a special rig.

Possible applications

If an exploration or appraisal well is continuously cored through the potential reservoir section, core analyses can give accurate information on sedimentological features, petrography and diagenetic processes. This can result in a much better understanding of the reservoir architecture and greatly improve the early estimates of production rates, ultimate recovery, etc.

Downhole data gathering

The cores, which may be up to 27 metres long, can reveal data about the formation which is not available through conventional well evaluation techniques. On-site core evaluation plays a key role in maximising the benefit of this data source, as core measured parameters and information on fluids can be affected by time and handling procedures. NMR/NML analysis of core, containing its original fluids, on the rig site is claimed to give a more accurate measure of porosity than measurements done some time later. Application of the MWD sonic tool, when it becomes available, will allow calibration of the well to seismic data in real drilling time. The existence of a core early in the exploration or appraisal process may advance regional geological knowledge, with a corresponding reduction in the uncertainty in HIIP and reserves. The core data can be supplemented by wireline logging and production tests.

Conclusions

In view of the current immaturity of this drilling technique, especially in softer formations, the use of the downsized conventional approach is considered preferable, unless there is an over-riding need for coring a significant proportion of the well. Use of the continuous coring technique by a number of operators is generating practical experience which tends to support this conclusion.